Rate constant and RRKM product study for the reaction between CH3 and C2H3at T=298 K

The total rate constant k(1) has been determined at P = 1 Torr nominal pres sure (He) and at T = 298 K for the vinyl-methyl cross-radical reaction: (1) CH3 + C2H3 --> Products. The measurements were performed in a discharge fl ow system coupled with collision-free sampling to a mass spectrometer opera ted at low electron energies. Vinyl and methyl radicals were generated by t he reactions of F with C2H4 and CH4, respectively. The kinetic studies were performed by monitoring the decay of C2H3 with methyl in excess, 6 < [CH3] (0)/ [C2H3](0) < 21. The overall rate coefficient was determined to be k(1) (298 K) = (1.02 +/- 0.53) X 10(-10) cm(3) molecule(-1) s(-1) with the quote d uncertainty representing total errors. Numerical modeling was required to correct for secondary vinyl consumption by reactions such as C2H3 + H and C2H3 + C2H3. The present result for k(1) at T = 298 K is compared to two pr evious studies at high pressure (100-700 Torr He) and to a very recent stud y at low pressure (0.9-3.7 Torr He). Comparison is also made with the rate constant for the similar reaction CH3 + C2H5 and with a value For k(1) esti mated by the geometric mean rule employing values for k(CH3 + CH3) and k(C2 H3 + C2H3). Qualitative product studies at T = 298 K and 200 K indicated fo rmation of C3H6, C2H2, and C3H5 as products of the combination-stabilizatio n, disproportionation, and combination-decomposition channels, respectively , of the CH3 + C2H3 reaction. We also observed the secondary C4H8 product o f the subsequent reaction of C3H5 with excess CH3; this observation provide s convincing evidence For the combination-decomposition channel yielding C3 H5 + H. RRKM calculations with helium as the deactivator support the presen t and very recent experimental observations that allylic C-H bond rupture i s an important path in the combination reaction. The pressure and temperatu re dependencies of the branching fractions are also predicted. (C) 2000 Joh n Wiley & Sons. Inc. Int J Chem Kinet 32: 104-316, 2000.